1. Field of the Invention
The present invention generally relates to an image scanning device, and more specifically to an image scanning device for enhancing accuracy of detecting a substance attached to a reflection side of a white reference board provided in the image scanning device.
2. Related Art
In an image scanning device (e.g., a copier), the amount of light to be scanned varies depending on the variation of the amount of light irradiated by a light source, variation of sensitivity of a photoelectric conversion part (e.g., a detection side of an image sensor), and the like. When the amount of light varies, quality of an image obtained by the scanning of a manuscript will be lowered. In response to this, shading correction processing is executed for inhibiting the low image quality. Here, the image scanning device is provided with a white reference board. The white reference board has a uniform white color, and is also referred to as “a shading board.” Specifically, the white reference board is oppositely disposed to the detection side of the image sensor.
The amount of light is generally corrected with the above-mentioned white reference board. A series of the correction processing is referred to as “the shading correction.” Specifically, two light sources are simultaneously turned on. The light sources are disposed in symmetrical positions, and the reflection side of the white reference board is interposed between them. With this arrangement, a scanning position is irradiated from both directions of the light sources. Then, the image sensor scans the light reflected by the white reference board as the white reference board data (hereinafter simply referred to as “white reference data”). Subsequently, a manuscript is scanned as image data. Next, the image data of the manuscript are converted while the white reference data are used as a reference of white color. Thus, the corrected image data of the manuscript are obtained. The corrected image data of the manuscript have an image with uniform brightness and sharpness, and deterioration of the image quality is maximally inhibited.
In the above-mentioned shading correction, the white reference data are used as a reference of white color. Accordingly, when a substance (e.g., a small piece of paper or dirt) attaches to the reflection side of the white reference board, data attributed to the attached substance will be included in the white reference data. When the shading correction is performed with the white data including the data attributed to the attached substance, a portion of the image will have abnormal density. Here, the portion with abnormal density corresponds to a portion of the reflection side of the white reference board with the attached substance. When the image is entirely seen, only the portion seems to have a line. As a result, image quality of the manuscript will be deteriorated, and it is difficult to obtain image data of a manuscript with sharpness.
In order to solve the above-mentioned problem, the conventional art has proposed a configuration with a roll shaped white reference board. In the art, when defect is generated in the white reference data because of the attached substance, the white reference board is rotated. A portion of the white reference board with the attached substance is thus prevented from being used as the reflection side of the white reference board. In short, the reflection side of the white reference board is changed. Accordingly, even when the substance is attached to the white reference board, the white reference data are allowed to be obtained through a portion of the white reference board without the attached substance, Deterioration of the image quality is thus prevented.
Also, Japanese Patent Publication No. 3581748 discloses a configuration that the accuracy of shading correction is determined with the white reference data based on the image data binarized with a predetermined threshold value. According to the publication, it is possible to determine the accuracy of shading correction with normal image processing steps performed by an image scanning device. Also, with the above-mentioned configuration, accuracy of shading correction is determined before the scanning of an image on the manuscript. Therefore, it is possible to prevent deterioration of image quality attributed to deterioration of an optical system, dirt, a scratch, and the like.
However, the above-mentioned conventional art is required to change the reflection side of the white reference board. Therefore, the number of components is increased and spaces for the components are necessary.
Also, according to the art disclosed in Japanese Patent Publication No. 3581748, even ignorable tiny specks of dirt will be detected when the threshold value is set too low. Accordingly, frequent cleaning is necessary for the white reference board. On the other hand, the attached substance is not detected when the threshold value is set too high. Accordingly, not detecting the attached substance causes deterioration in the image quality. In short, without an appropriate setting for the threshold value, chances of erroneously determining accuracy of shading correction will be higher. As a result, it is impossible to prevent appropriately deterioration of the image quality.
Furthermore, the above-mentioned arts are configured to determine if the attached substance exists based on the white reference data. However, the white reference data are obtained when the amount of irradiated light is extremely large. Therefore, there is little difference between the amount of reflected light from the white reference board and the amount of reflected light from the attached substance. Accordingly, there is a limitation in the detection accuracy of the attached substance.
Accordingly, aspects of the present invention have been created to solve the above-mentioned problems occurring in the conventional practice, and to provide an image scanning device for enhancing accuracy of detecting substance attached to a reflection side of a white reference board provided in the image scanning device.